1. Field of the Invention
The present invention relates to a bump-attached wiring circuit board having bumps of a uniform height, and to a method for manufacturing this board.
2 Description of the Related Art
It is common practice to make connections with microscopic bumps (such as those with a diameter of 50 μm and a height of 30 μm) when connecting a wiring circuit board and electronic elements such as semiconductor elements or liquid crystal display elements, or when connecting layers in a multilayer wiring board.
FIGS. 3A to 3E illustrate a typical method for forming bumps of this size.
First, as shown in FIG. 3A, a two-layer flexible board 33 is prepared by affixing a copper foil 32 to a polyimide film 31, and the copper foil 32 is patterned by photolithography to form a wiring circuit 34 (FIG. 3B).
Next, a cover coat layer 35 is formed by a conventional method on the wiring circuit 34 (FIG. 3C). For instance, a polyamic acid layer may be formed on the wiring circuit 34 and patterned by photolithography, and a cover coat layer 35 formed by imidation. Alternatively, a resist ink may be applied by printing.
Next, bump holes 36 are formed by irradiation with laser light in the region of the polyimide film 31 corresponding to the wiring circuit 34 (FIG. 3D), and then the cover coat layer 35 is covered with a protective film (not shown) as needed, after which microscopic bumps are formed by growing metal bumps 37 over the wiring circuit 34 exposed at the bottom of the bump holes 36 (FIG. 3E).
However, when the bump holes 36 are made by irradiation with laser light, there is variance in their open surface area due to variance in the smear amount clinging to the bottom of the bump holes 36, and this results in the problem of considerable variance occurring in the height of the metal bumps 37. This makes it difficult to achieve stable bump connection. It is particularly difficult to connect semiconductor elements all at once to a wiring circuit by ultrasonic connection. A plating pretreatment is also essential in order to improve the adhesive strength between the wiring circuit 34 and the metal bumps 37 formed on it.